A known image processing method for imaging apparatuses such as digital cameras is such that a subject image is formed on an image sensor through a lens, and this image sensor conducts photoelectric conversion of the subject image so as to generate an image signal.
Recent imaging apparatuses such as digital cameras often have moving image capture capability in addition to still image capture capability. On the other hand, video cameras are often required to have high-definition still image capture capability. As such, imaging apparatuses such as digital cameras and video cameras are required to have both high-definition still image capture capability and high-definition (e.g. hi-vision) moving image capture capability.
Such imaging apparatus which can capture both still images and moving images are typically arranged so that, while incase of still image capture a high-definition image is generated by reading out signals from all pixels, in case of moving image capture an image is generated by reading out signals from a limited number pixels rather than from all pixels, for the purpose of high-speed readout and less power consumption.
For example, a 8M-pixel image sensor (so-called solid-state image sensor) is typically used for still image capture. In this regard, provided that the depth of each pixel is 12 bits, the image sensor (i.e. the solid-state image sensor of the present invention) is required to output information of 8M pixel*12 bit/pix=96 Megabits.
While the time required for output is not really a problem in still image capture, moving image capture involving moving image readout at a high-speed frame rate of for example 30 fps requires the image sensor to output information of 96 Mbit/frame*30 frame/sec=2.88 Gbps, when signals are obtained from all pixels of the 8M-pixel image sensor. Since such an output data rate is unrealistic and consumes a considerable amount of power, moving image capture requires to limit the number of pixels of the image sensor from which signals are output.
A known method to limit the number pixels from which outputs of the image sensor are made is cut-out output in which a part of a screen is cut out and output.
For example, in the cut-out output, an image sensor which uses 8 megapixels for a still image outputs a 2-megapixel moving image in such a way that a central part of the screen which is a quarter of the entire screen is cut out and output. This method is disadvantageous in that the angle of view in moving image capture is narrower than that of still image capture.
Other known methods to limit the number of pixels output from an image sensor are decimation output in which signals output from pixels are decimated and pixel mixture output in which plural pixel signals of the same color are mixed with one another and output (see e.g. Patent Document 1 (Japanese Unexamined Patent Publication No. 2007-088733)). For example, a signal for one pixel is output from 2×2 pixels aligned in vertical and horizontal directions by performing decimation or pixel mixture, with the result that the output data rate is reduced to ¼.
For example, as shown in FIG. 9, a color image sensor having Bayer-arranged color filters of R (red), G (green), and B (blue) primary colors is arranged so that G output pixels are disposed in a checkerboard pattern, and a row in which G output pixels and R output pixels are alternated and a row in which B output pixels and G output pixels are alternated are alternated. A pixel mixture output of this case is generated in such a way that pixel signals of 2×2 pixels surrounding a target pixel are added up so that a single signal is generated.
More specifically, in case of a Bayer-arranged pixel group in FIG. 9(a), signals are obtained from 4 pixels of the same color, which are vertical two pixels sandwiching another pixel therebetween and horizontal two pixels sandwiching another pixel therebetween. The signals are mixed and added up as shown in (b), and pixel data is reduced as shown in (c) and output.